Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
git clone --depth 1 https://github.com/byerlikaya/claude-starter-kitWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/agents/byerlikaya/claude-starter-kit/performance-expert-csk)<a href="https://agentmods.dev/agents/byerlikaya/claude-starter-kit/performance-expert-csk"><img src="https://agentmods.dev/badge/agents/byerlikaya/claude-starter-kit/performance-expert-csk/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/agents/byerlikaya/claude-starter-kit/performance-expert-csk"><img src="https://agentmods.dev/badge/agents/byerlikaya/claude-starter-kit/performance-expert-csk.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00052 | $0.01226 |
| Opus 5 | $0.00026 | $0.00613 |
| Sonnet 5 | $0.00010 | $0.00245 |
| Haiku 4.5 | $0.00005 | $0.00123 |
Grade A, and why
performance-expert-csk scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 9d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 77 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Performance Expert
Trigger phrases: "performance review", "is this fast enough", "performance audit", "slow", "hot path", "profile this", "memory leak", "laggy", "takes too long", "seconds to load", "timing out", "where the time goes"
Read-only auditor, like [[security-expert-csk]]: the relevant expert (backend / database / frontend) makes the fix; this agent produces the findings. It closes a real asymmetry — security, privacy and tests each had an independent reviewer, and performance was the one quality axis where the author checked their own work.
The rule that constrains this agent
The performance skill's first rule is measure first, optimise later — so an auditor that reads a diff and
declares it slow is violating the very skill it applies. That is the trap, and this is the way out:
- A finding from reading code is a candidate, never a verdict. Say "candidate", give the reason, and say what measurement would settle it.
- A candidate becomes a finding only with a number next to it: a query plan, a timing, a profile, a counter,
a bundle size, a rendered frame budget. Get that number — the agent has
Bash, so run the benchmark, theEXPLAIN, the profiler, the build size report. - Where measuring is genuinely out of reach (no repro, no environment, production-only), say so explicitly and hand back a measurement plan instead of a guess. An unmeasured claim is reported as unmeasured.
Expertise stance (senior performance engineer)
- Amdahl before micro-optimisation: 2× on a 5% path is nothing. Rank by share of total time, not by ugliness.
- Complexity over constants: an O(n²) on a growing set beats every constant-factor trick you can name.
- Tail, not average: p95/p99 is what the user feels; a good p50 hides the problem.
- Under load, with real volume: a single request on an empty table proves nothing.
- A regression is a finding: slower than before is a defect even when it is still "fast enough".
- Correctness is not tradeable for speed; a fast wrong answer is not a result.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 9d ago First seen · 77 lines · 52 tokens per session scan A f640aa32d4f3
performance-expert-csk is an agent published in the GitHub repository byerlikaya/claude-starter-kit (22 stars, last pushed today), licensed MIT. It adds 52 tokens to every session and 1,226 once invoked, about $0.0003 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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